In a water treatment process, an inclined tube in a mechanical accelerated clarification basin and a corrugated plate in a horizontal flow sedimentation tank have hydrodynamic characteristics, which are used to raise slowly sewage containing active sludge along the inclined tube or the corrugated plate through the specially designed gap space. While raising and translating, the sludge absorbs gradually impurities in water treated by adding a treatment reagent. When the density of the sludge is greater than that of water and sufficient to overcome the lift effect of the rising water, the sludge will settle slowly along the surface or gap of the inclined tubes or the corrugated tubes and slide down into a sludge collecting pond at the bottom. The clarified water rises slowly along the inclined tubes or the corrugated plates through the specially designed gap space to a catchment trough to flow away, thereby the purpose for separating clean water from turbid precipitates is achieved. It can be seen in FIG. 1 the clarification principle of the inclined tubes in the mechanical accelerated clarification basin and the corrugated plates in the horizontal flow sedimentation tank.
Whatever material an inclined tube or corrugated plate is made from, its friction coefficient can not be zero. Therefore when the inclined tube or corrugated plate is used, gradually clogging will occur and become a problem, which causes unsatisfied effect on sludge sliding. For example, after the inclined tube in the mechanical accelerated clarification basin is used for a period of time, the sludge settles on the surface of each inclined tube unevenly. Especially in spring and summer, algae in water propagate prolifically with abundant sunshine. The algae propagated attach to the surface of the inclined tube, which increases the friction coefficient of the inclined tube and decreases the flowability of the sludge settled on the surface of the inclined tube, thereby destroying the hydrodynamic characteristics of the inclined tube. The sludge settled on the surface of the inclined tube is then over-accumulated gradually and cannot slide down, and thus in the inclined tube the space through which cleaned water slowly raises will be gradually clogged by the sludge and algae.
The clogging phenomenon is a progressively developing process. While the total amount of water treated is constant, clogging on a portion of the inclined tube(s) will definitely increase the flow rate of the slowly rising water per unit area above the other portions of the inclined tube(s), thereby changing the hydrodynamic characteristics of the inclined tube(s). When the flow rate of the water increases up to a certain limit (for example, generally 2 mm/sec to 3.0 mm/sec), the requirements for separating clean water from turbid precipitates can not be met. The water with the flow rate increased per unit area will finally flush back up the floccules with the density slightly greater than that of water and the sludge settled on the inclined tube(s) or the corrugated plate(s) from the gap or the inside of the inclined tube, leading to the sediment upturning phenomenon in which the floccules float up to mix with clean water. The mixture flows into a catchment trough that collects clean water, and reaches the next filter tank, so that the clarification basin completely loses its function of separating water from sludge to clarify water. It can be seen in FIG. 2 for the clogging process of the inclined tube(s) in the mechanical accelerated clarification basin or the corrugated plate(s) in the horizontal flow sedimentation tank.
Taking the mechanical accelerated clarification basin as an example, after many years of observation and research, we found the following conclusions. It can be seen in FIG. 3 the relationship between the water treatment capacity of the mechanical accelerated clarification basin and the degree of clogging in the inclined tube(s) (or the corrugated plate(s)). As shown in the figure, the water treatment capacity of the mechanical accelerated clarification basin is inversely proportional to the degree of clogging in the inclined tube (or the corrugated plate), that is, the higher the clogging rate of the inclined tube (or the corrugated plate), the lower its water treatment capacity. As the operation time of the mechanical accelerated clarification basin goes, the clogging rate of the inclined tube (or the corrugated plate) increases continuously, while its water treatment capacity decreases continuously. Therefore, in order to ensure the quality of effluent water from the mechanical accelerated clarification basin, the amount of water treated has to be decreased continuously. Thus, the mechanical accelerated clarification basin generally operates within its design capacity, and its actual treatment capacity decreases gradually. Many mechanical accelerated clarification basins practically operate at levels substantively lower than their design capacities, which is the result of their continuously decreasing thresholds (its actual treatment capacities). The threshold described herein represents the line between two areas shown in FIG. 3, a working area with unsatisfied clarification effect and a working area with satisfied clarification effect. In order to ensure the quality of the effluent water, it is generally to control the treatment amount of water lower than the threshold.
If the treatment amount of water is higher than the threshold, that is, the flow rate of the rising water in the inclined tube (or the corrugated plate) exceeds the design flow rate (for example, exceeds 2.0 mm/sec to 3.0 mm/sec), then a great deal of floccules in the inclined tube will overflow the surface of the inclined tube, the mechanical accelerated clarification basin will lose clarification effect, and the water containing a great deal of floccules will flow to the subsequent filter tank through the catchment trough, thereby causing a great pressure for the filter tank. The pores between filter materials used for filtering will be clogged. Now this problem described above can be solved only by timely backwashing the filter tank and decreasing the amount of water in the mechanical accelerated clarification basin, which, however, will further consume a great amount of water. After a long-term observation, we found that the threshold of the water treatment capacity in the mechanical accelerated clarification basin is a variable which is affected by many factors, such as temperature, water quality, the treatment amount of water, etc. The most important issue is the clogging rate of the inclined tube (or the corrugated plate).
In actual operation, for the sake of furthest ensuring the water treatment capacity in the mechanical accelerated clarification basin, its working point (i.e. the treatment amount of water) is generally adjusted to a level slightly lower than the threshold so that the floccules will not overflow the surface of the inclined tube. However, here the actual treatment capacity of the mechanical accelerated clarification basin is lower than the designed treatment capacity. As the operation time elapses, its actual treatment capacity will further decrease. Therefore a good deal of floccules will overflow from the inclined tube due to the continuous decreasing of the threshold, even if the treatment amount of water is not changed in the process of operation. This is the basic reason why the mechanical accelerated clarification basin cannot operate steadily over a long period of time. In a word, the inherent defect in the design structure of the mechanical accelerated clarification basin brings great difficulty for managing the water treatment process. This problem has not been solved since the first mechanical accelerated clarification basin in the world began to operate.
The clogging rate is generally measured and adjusted manually in the country, which brings great work stress for operators on duty. Especially in the night when the illumination is not good at the location, it is not easy to find the treatment amount of water exceeding the threshold in time, which causes great wastes of water and a treatment reagent. While the monitoring and controlling levels in the developed countries are much higher than ours, there is no good method for solving this problem radically until now.
When the above phenomenon occurs, that is, the water treatment capacity decreases to a certain degree, in order to ensure the water treatment capacity, the normal way is to stop the operation to drain water entirely, wash the inclined tubes one by one with the high pressure water gun and clean down the clogged sludge and algae completely, and then resume operation. The washing period may be more than ten days or as less as six or seven days. While each washing, a great amount of water with a treatment reagent is drained in vain, and a certain amount of the treated water has to be used in washing process, which causes enormous wastes. According to statistics, typically a water treatment plant will self-consume water about 15% in the production process, among which the self-consumed water resulting from the problem described above will be conservatively estimated to be about 3˜5% or even more. Because the mechanical accelerated clarification basin is in the first step of water treatment process, if the quality of its effluent water cannot be guaranteed, it certainly will bring about tremendous pressure for the next treatment process. This is a bottleneck for the water treatment plant in water production process.
Other than the large consumption on manpower and material resources, the method of manual washing can not guarantee a thorough washing and cleaning too. Some dead comers may be always left unwashed, which reduces the effective area of passage of the inclined tube or the corrugated plate. Furthermore, if the process of stopping and washing is not performed frequently, the overweight of the settled sludge will inevitably speeded up the aging and damage of the inclined tube or the corrugated plate, and may cause the supporter supporting the inclined tube and the corrugated plate to be collapsed due to the settled sludge. Additionally, workers' walking ceaselessly on the inclined tube when doing the washing may also causes certain extent of deformation on the surface of the inclined tube, thereby shortening the service life of the inclined tube and aggravating the clogging of the inclined tube.
In conclusion, the gradual clogging caused by the settled sludge in the sedimentation device in these water treatment systems, such as a mechanical accelerated clarification basin or horizontal flow sedimentation tank, has become a difficult problem which perplexes water supply enterprises for a long time, and it is a bottleneck in the water treatment process for each water treatment plant. In order to solve the problem of the settled sludge clogging gradually in the inclined tube (or on the corrugated plate), many methods have been thought of, such as replacing the glass reinforced plastic material of the inclined tube with copolymer material so as to accelerate the sliding of the settled sludge by reducing the friction force, etc. However, the effects are not satisfied. Some researches on this problem were in some articles in water supply journals, but a technical scheme which can thoroughly solve this worldwide problem has not yet been seen so far.